The present invention generally relates to processes for making electrical connections to semiconductor integrated circuit chips and, more specifically, to such processes of the controlled collapse chip connection (C4) type.
The use of C4 or solder bump terminals on semiconductor devices for face-down or "flip chip" bonding to a supporting dielectric substrate or module is well known in the art. U.S. Pat. No. 4,434,434 issued to Somnath Bhattacharya et al on Feb. 28, 1984 and other patents cited therein describe the C4 technique in detail. Briefly, in accordance with one approach, solder balls are connected via solder bump terminals and contact pads to semiconductor devices which normally are passivated with a brittle coating such as silicon dioxide. Each said terminal is located at a contact opening, extending through the passivating layer, previously coated with a solder bump contact pad. Each pad extends through the opening and connects with the underlying device circuitry. The contact pads comprise metallic laminates such as chromium, copper-chromium and copper.
As pointed out in the cited patent, the BLM technique has been used extensively but a tendency has been noted to exist, especially when an approximately 95% Pb-5% Sn solder composition is used for the solder ball, for the brittle passivating layer to crack about the perimeter of the solder ball. Stress appears to develop at the abrupt edge of the contact pad, causing the brittle passivating layer to crack around the solder ball. The cited patent further teaches that the foregoing tendency to crack can be reduced by delocalizing or spreading out the solder edge stresses imposed on the contact pad structure and the underlying brittle passivating layer.
The stresses are spread out by modifying the geometry of the laminated contact pad to incorporate a graded or stepped profile at the peripheral portion of the pad, i.e., a bottom chromium layer of greater lateral extent, a top copper layer of lesser lateral extent and an intermediate layer of an intermix of the copper and chromium. The differing lateral extents of the chrome and copper layers are achieved, in accordance with the cited patent, by using a rotating dome vacuum evaporator to deposit the layers while off-centering the chromium source during the respective evaporation cycles. No other method is disclosed, for producing the graded or stepped profile defined above.